More Than You Ever Wanted
to Know About Air Conditioners.
Understanding Air Conditioners
Many people buy or use air conditioners without understanding their designs,
components, and operating principles. Proper sizing, selection, installation,
maintenance, and correct use are keys to cost-effective operation and
lower overall costs.
How Air Conditioners Work
Air conditioners employ the same operating principles and basic components
as your home refrigerator. An air conditioner cools your home with a cold
indoor coil called the evaporator. The condenser, a hot outdoor coil,
releases the collected heat outside. The evaporator and condenser coils
are serpentine tubing surrounded by aluminum fins. This tubing is usually
made of copper. A pump, called the compressor, moves a heat transfer fluid
(or refrigerant) between the evaporator and the condenser. The pump forces
the refrigerant through the circuit of tubing and fins in the coils. The
liquid refrigerant evaporates in the indoor evaporator coil, pulling heat
out of indoor air and thereby cooling the home. The hot refrigerant gas
is pumped outdoors into the condenser where it reverts back to a liquid
giving up its heat to the air flowing over the condenser's metal tubing
and fins.
Types of Air Conditioners
There are three basic types of air conditioners: room air conditioners,
split-system central air conditioners, and packaged central air conditioners.
1) Room Air Conditioners
Room air conditioners cool rooms rather than the entire home. If they
provide cooling only where they're needed, room air conditioners are
less expensive to operate than central units, even though their efficiency
is generally lower than that of central air conditioners.
~Central Air Conditioners
Central air conditioners circulate cool air through a system of supply
and return ducts. Supply ducts and registers (i.e., openings in the
walls, floors, or ceilings covered by grills) carry cooled air from
the air conditioner to the home. This cooled air becomes warmer as
it circulates through the home; then it flows back to the central
air conditioner through return ducts and registers. A central air
conditioner is either a split-system unit or a packaged unit.
2)Split-system Air Conditioners
In a split-system central air conditioner, an outdoor metal cabinet
contains the condenser and compressor, and an indoor cabinet contains
the evaporator. In many split-system air conditioners, this indoor cabinet
also contains a furnace or the indoor part of a heat pump. The air conditioner's
evaporator coil is installed in the cabinet or main supply duct of this
furnace or heat pump. If your home already has a furnace but no air
conditioner, a split-system is the most economical central air conditioner
to install.
3)Package Central Air Conditioner
In a packaged central air conditioner, the evaporator, condenser, and
compressor are all located in one cabinet, which usually is placed on
a roof or on a concrete slab next to the house's foundation. This type
of air conditioner also is used in small commercial buildings. Air supply
and return ducts come from indoors through the home's exterior wall
or roof to connect with the packaged air conditioner, which is usually
located outdoors. Packaged air conditioners often include electric heating
coils or a natural gas furnace. This combination of air conditioner
and central heater eliminates the need for a separate furnace indoors.
Maintaining Existing Air Conditioners
Older air conditioners may still be able to offer years of relatively
efficient use. However, making your older air conditioner last requires
you to perform proper operation and maintenance.
Air Conditioning Problems
One of the most common air conditioning problems is improper operation.
If your air conditioner is on, be sure to close your home's windows and
outside doors.
Other common problems with existing air conditioners result from faulty
installation, poor service procedures, and inadequate maintenance. Improper
installation of your air conditioner can result in leaky ducts and low
air flow. Many times, the refrigerant charge (the amount of refrigerant
in the system) does not match the manufacturer's specifications. If proper
refrigerant charging is not performed during installation, the performance
and efficiency of the unit is impaired. Service technicians often fail
to find refrigerant charging problems or even worsen existing problems
by adding refrigerant to a system that is already full. Air conditioner
manufacturers generally make rugged, high quality products. If your air
conditioner fails, it is usually for one of the common reasons listed
below:
~Refrigerant leaks: If your air conditioner is low on refrigerant,
it was either undercharged at installation, or it leaks. If it leaks,
simply adding refrigerant is not a solution. A trained technician should
fix any leak, test the repair, and then charge the system with the correct
amount of refrigerant. Remember that the performance and efficiency
of your air conditioner is greatest when the refrigerant charge exactly
matches the manufacturer's specification, and is neither undercharged
nor overcharged.
~Inadequate maintenance. If you allow filters and air conditioning
coils to become dirty, the air conditioner will not work properly, and
the compressor or fans are likely to fail prematurely.
electric control failure. The compressor and fan controls can wear out,
especially when the air conditioner turns on and off frequently, as
is common when a system is oversized. Because corrosion of wire and
terminals is also a problem in many systems, electrical connections
and contacts should be checked during a professional service call.
~Regular Maintenance. An air conditioner's filters, coils,
and fins require regular maintenance for the unit to function effectively
and efficiently throughout its years of service. Neglecting necessary
maintenance ensures a steady decline in air conditioning performance
while energy use steadily increases.
Air Conditioner Filters
The most important maintenance task that will ensure the efficiency of
your air conditioner is to routinely replace or clean its filters. Clogged,
dirty filters block normal air flow and reduce a system's efficiency significantly.
With normal air flow obstructed, air that bypasses the filter may carry
dirt directly into the evaporator coil and impair the coil's heat-absorbing
capacity. Filters are located somewhere along the return duct's length.
Common filter locations are in walls, ceilings, furnaces, or in the air
conditioner itself. Some types of filters are reusable; others must be
replaced. They are available in a variety of types and efficiencies. Clean
or replace your air conditioning system's filter or filters every month
or two during the cooling season. Filters may need more frequent attention
if the air conditioner is in constant use, is subjected to dusty conditions,
or you have fur-bearing pets in the house.
Air Conditioner Coils
The air conditioner's evaporator coil and condenser coil collect dirt
over their months and years of service. A clean filter prevents the evaporator
coil from soiling quickly. In time, however, the evaporator coil will
still collect dirt. This dirt reduces air flow and insulates the coil
which reduces its ability to absorb heat. Therefore, your evaporator coil
should be checked every year and cleaned as necessary. Outdoor condenser
coils can also become very dirty if the outdoor environment is dusty or
if there is foliage nearby. You can easily see the condenser coil and
notice if dirt is collecting on its fins. You should minimize dirt and
debris near the condenser unit. Your dryer vents, falling leaves, and
lawn mower are all potential sources of dirt and debris. Cleaning the
area around the coil, removing any debris, and trimming foliage back at
least 2 feet (0.6 meters) allow for adequate air flow around the condenser.
Coil Fins
The aluminum fins on evaporator and condenser coils are easily bent and
can block air flow through the coil. Air conditioning wholesalers sell
a tool called a "fin comb" that will comb these fins back into
nearly original condition.
Sealing and Insulating Air Ducts
An enormous waste of energy occurs when cooled air escapes from supply
ducts or when hot attic air leaks into return ducts. Recent studies indicate
that 10% to 30% of the conditioned air in an average central air conditioning
system escapes from the ducts. For central air conditioning to be efficient,
ducts must be airtight. Hiring a competent professional service technician
to detect and correct duct leaks is a good investment, since leaky ducts
may be difficult to find without experience and test equipment. Ducts
must be sealed with duct "mastic." The old standby of duct tape
is ineffective for sealing ducts. Obstructions can impair the efficiency
of a duct system almost as much as leaks. You should be careful not to
obstruct the flow of air from supply or return registers with furniture,
drapes, or tightly fitted interior doors. Dirty filters and clogged evaporator
coils can also be major obstructions to air flow.The large temperature
difference between attics and ducts makes heat conduction through ducts
almost as big a problem as air leakage and obstructions. Ducts in attics
should be insulated heavily in addition to being made airtight.
Buying New Air Conditioners
Today's best air conditioners use 30% to 50% less energy to produce the
same amount of cooling as air conditioners made in the mid 1970s. Even
if your air conditioner is only 10 years old, you may save 20% to 40%
of your cooling energy costs by replacing it with a newer, more efficient
model.
Sizing Air Conditioners
Air conditioners are rated by the number of British Thermal Units (Btu)
of heat they can remove per hour. Another common rating term for air conditioning
size is the "ton," which is 12,000 Btu per hour.
The size of an air conditioner depends on:
~how large your home is and how many windows it has;
~how much shade is on your home's windows, walls, and roof;
~how much insulation is in your home's ceiling and walls;
~how much air leaks into your home from the outside; and
~how much heat the occupants and appliances in your home generate.
An air conditioner's efficiency, performance, durability, and initial
cost depend on matching its size to the above factors.
Make sure that you buy the correct size air conditioner. Two groups—the
Air Conditioning Contractors of America (ACCA) and the American Society
of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE)—publish
calculation procedures for sizing central air conditioners. Reputable
air conditioning contractors will use one of these procedures, often performed
with the aid of a computer, to size your new central air conditioner.
Be aware that too large of an air conditioner will not provide the best
cooling. Buying an oversized air conditioner penalizes you in the following
ways.
It costs more to buy a larger air conditioner than you need.
The larger-than-necessary air conditioner cycles on and off more frequently,
reducing its efficiency. Frequent cycling makes indoor temperatures
fluctuate more and results in a less comfortable environment. Frequent
cycling also inhibits moisture removal. In humid climates, removing
moisture is essential for acceptable comfort. In addition, this cycling
wears out the compressor and electrical parts more rapidly.
A larger air conditioner uses more electricity and creates added demands
on electrical generation and delivery systems.
Air Conditioner Efficiency
Each air conditioner has an energy-efficiency rating that lists how many
Btu per hour are removed for each watt of power it draws. For room air
conditioners, this efficiency rating is the Energy Efficiency Ratio, or
EER. For central air conditioners, it is the Seasonal Energy Efficiency
Ratio, or SEER. These ratings are posted on an Energy Guide Label, which
must be conspicuously attached to all new air conditioners. Many air conditioner
manufacturers are participants in the voluntary EnergyStar® labeling
program (see Source List in this publication). EnergyStar-labeled appliances
mean that they have high EER and SEER ratings.
In general, new air conditioners with higher EERs or SEERs sport higher
price tags. However, the higher initial cost of an energy-efficient model
will be repaid to you several times during its life span. Your utility
company may encourage the purchase of a more efficient air conditioner
by rebating some or all of the price difference. Buy the most efficient
air conditioner you can afford, especially if you use (or think you will
use) an air conditioner frequently and/or if your electricity rates are
high.
Room Air Conditioners—EER
Room air conditioners generally range from 5,500 Btu per hour to 14,000
Btu per hour. National appliance standards require room air conditioners
built after January 1, 1990, to have an EER of 8.0 or greater. Select
a room air conditioner with an EER of at least 9.0 if you live in a mild
climate. If you live in a hot climate, select one with an EER over 10.
The Association of Home Appliance Manufacturers reports that the average
EER of room air conditioners rose 47% from 1972 to 1991. If you own a
1970s-vintage room air conditioner with an EER of 5 and you replace it
with a new one with an EER of 10, you will cut your air conditioning energy
costs in half.
Central Air Conditioners—SEER
National minimum standards for central air conditioners require a SEER
of 9.7 and 10.0, for single-package and split-systems, respectively. But
you do not need to settle for the minimum standard—there is a wide
selection of units with SEERs reaching nearly 17. Before 1979, the SEERs
of central air conditioners ranged from 4.5 to 8.0. Replacing a 1970s-era
central air conditioner with a SEER of 6 with a new unit having a SEER
of 12 will cut your air conditioning costs in half.
Hiring Professional Service
When your air conditioner needs more than the regular maintenance described
previously, hire a professional service technician. A well-trained technician
will find and fix problems in your air conditioning system. However, not
all service technicians are competent. Incompetent service technicians
forsake proper diagnosis and perform only minimal stop-gap measures. Insist
that the technician:
~check for correct amount of refrigerant;
~test for refrigerant leaks using a leak detector;
~capture any refrigerant that must be evacuated from the system, instead
of illegally releasing it to the atmosphere;
~check for and seal duct leakage in central systems;
~measure air flow through the evaporator coil;
~verify the correct electric control sequence and make sure that the
heating system and cooling system cannot operate simultaneously;
~inspect electric terminals, clean and tighten connections, and apply
a non-conductive coating if necessary;
~oil motors and check belts for tightness and wear; and
~check the accuracy of the thermostat.
Choosing a Contractor
Choosing a contractor may be the most important and difficult task in
buying a new central air conditioning system. Ask prospective contractors
for recent references. If you are replacing your central air conditioner,
tell your contractor what you liked and did not like about the old system.
If the system failed, ask the contractor to find out why. The best time
to fix existing problems is when a new system is being installed.
When designing your new air conditioning system,
the contractor should:
~use a computer program or written calculation procedure to size the
air conditioner;
~provide a written contract listing the main points of your installation
that includes the results of the cooling load calculation;
~give you a written warranty on equipment and workmanship; and
~allow you to hold the final payment until you are satisfied with the
new system.
***Avoid making your decision solely on the basis of price. The quality
of the installation should be your highest priority because quality will
determine energy cost, comfort, and durability.
Installation and Location of Air Conditioners
If your air conditioner is installed correctly, or if major installation
problems are found and fixed, it will perform efficiently for years with
only minor routine maintenance. However, many air conditioners are not
installed correctly. As an unfortunate result, modern energy-efficient
air conditioners can perform almost as poorly as older inefficient models.
Be sure that your contractor performs the following procedures when installing
a new central air conditioning system:
~allows adequate indoor space for the installation, maintenance, and
repair of the new system, and installs an access door in the furnace
or duct to provide a way to clean the evaporator coil.
~uses a duct-sizing methodology such as the Air Conditioning Contractors
of America (ACCA) Manual D.
ensures there are enough supply registers to deliver cool air and enough
return air registers to carry warm house air back to the air conditioner.
~installs duct work within the conditioned space, not in the attic,
wherever possible.
~seals all ducts with duct mastic and heavily insulates attic ducts.
~locates the condensing unit where its noise will not keep you or your
neighbors awake at night, if possible.
~places the condensing unit in a shady spot, if possible, which can
reduce your air conditioning costs by 1% to 2%.
~verifies that the newly installed air conditioner has the exact refrigerant
charge and air flow rate specified by the manufacturer.
~locates the thermostat away from heat sources, such as windows, or
supply registers.
***If you are replacing an older or failed split system, be sure that
the evaporator coil is replaced with a new one that exactly matches
the condenser coil in the new condensing unit. (The air conditioner's
efficiency will likely not improve if the existing evaporator coil is
left in place; in fact, the old coil could cause the new compressor
to fail prematurely.)
If you install a new room air conditioner, try to:
~locate the air conditioner in a window or wall area near the center
of the room and on the shadiest side of the house.
~minimize air leakage by fitting the room air conditioner snugly into
its opening and sealing gaps with a foam weatherstripping material.
~Paying attention to your air conditioning system saves you money and
reduces environmental pollution. Notice whether your existing system
is running properly, and maintain it regularly. Or, if you need to purchase
a new air conditioner, be sure it is sized and installed correctly and
has a good EER or SEER rating.
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